Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are lighter and have higher energy densities than existing batteries, and can therefore be advantageously used in power sources for propelling vehicles such as electric vehicles and hybrid vehicles or in so-called portable power sources for personal computers, hand-held devices and the like.
Such batteries are used in a state whereby the voltage is generally controlled so as to be held within a prescribed range (for example, 3.0 to 4.1 V), but if a higher voltage than normal is supplied due to improper operation or the like, the voltage may exceed the prescribed voltage and overcharging may occur. If overcharging occurs, gases may be generated due to the non-aqueous electrolyte decomposing and the temperature inside the battery may rise due to heat generated by the active material.
As technique for solving this problem, Patent Literature 1 to 3, for example, have disclosed adding an aromatic compound, which has an oxidation potential that is higher than the maximum charging potential of a positive electrode, to a non-aqueous electrolyte. When the battery reaches an overcharged state, the aromatic compound is electrochemically polymerized and a high resistance coating film is formed on the surface of the positive electrode. In this way, migration of lithium ions is suppressed and development of the overcharging can be halted.